This invention relates to a method and apparatus of joining of thermoplastic materials.
While metals are currently being used for many applications, and new metal alloys are being developed, more parts are being designed and manufactured using plastic and composite materials. Likewise, existing parts that are currently made with metals, are being redesigned and manufactured with plastic materials.
Plastics offer several advantages over metals. Engineering plastics have a higher strength to weight ratio than many metals. As a result, they require less energy to move due to decreased inertia, which results in energy savings and faster moving parts. Likewise, while most metals are isotropic, composite materials can be designed to be anisotropic. This allows designers to use less material in unloaded areas while strategically placing material in areas under high loads and stresses resulting in even higher strength to weight ratios.
Plastics are tough, viscoelastic materials. Because of this, most plastics can undergo high amounts of deformation before yielding. Durability is a definite strength when dealing with plastics.
Plastics are more resistant to the environment than most other classes of materials. This is why many chemical containers are made of plastics because they are chemical resistant, lightweight, and will not break if dropped. Fillers and additives, such as chlorine and UV stabilizers, can be added to raw plastic materials to improve their environmental resistance.
Plastics are also easily processed. Thermoplastic materials must simply be melted, molded into a desired shape, and then allowed to cool. Injection molding is a simple process that has revolutionized several industries by making it possible to produce complex parts in large volumes, at low cost. Costs are reduced further as scrap material can be easily recycled, sometimes even as part of the production process.
Despite the continuous improvements and development of new plastic materials, the processes used to fabricate final products are often inadequate.
One of the areas undergoing the greatest advances in the past few years is the area of joining processes. While, several new processes have been developed to meet the need for producing large volumes of quality plastic parts, there is still a need for a process for joining plastic materials, particularly for structural and high performance applications. Unfortunately, plastics can be difficult to join due to their low surface energies, poor wetability, and the presence of release agents from previous processing steps. A good joining method should be able to meet basic requirements. These requirements may include:
1. Reproducibility of joint efficiency
2. Ability to join materials in various joint geometries
3. Suitability for small and large bonding areas
4. Minimal surface preparation required
5. Minimal use of expensive specialty equipment
6. Potential for production applications
7. Retention of joint integrity in a variety of environments and load systems
Modern joining methods address these process requirements in a variety of ways. Modern joining methods fall into two general categories: mechanical fastening and bonding. Mechanical fastening utilizes a separate, mechanical device that holds the materials together at the joint area. Bonding joins parts through adhesives or fusion (welding). Adhesive bonding includes the use of adhesives or solvents to chemically join parts. Welding includes thermal, friction/mechanical, and electromagnetic methods that melt or soften to fuse the material together at the joint. While all of these methods have advantages in joining plastics, and all have been used successfully in commercial applications, there is still a need for a method for joining plastics. As discussed in greater detail below, existing methods still are deficient for the joining the plastics.
Mechanical Fastening
Mechanical fastening involves attaching or joining parts by using external materials or components to mechanically hold the plastic parts together. Screws, rivets, clips, and brackets are some examples of mechanical fasteners, which can be metal or plastic, depending on the application. A disadvantage of mechanical fasteners is the concentration of stresses that develop at the localized fastening areas. Although straightforward and relatively simple to form, mechanically fastened joints are typically of low performance, when compared with other methods
Adhesive Bonding
Adhesives
Adhesive bonding is a mature process. Adhesive research is very advanced and has made the joining of virtually any material possible. Adhesives effectively join a variety of plastics, and are relatively simple, requiring neither expensive equipment nor extensive training of personnel.
Because most adhesives rely on chemical reactions between materials to effectively bond parts, there are disadvantages associated with this process. The performance of adhesive joints depends greatly on part surface preparation, especially with thermoplastics, to ensure that all release agents have been removed prior to bonding. Surface preparation and long cure times reduce the applicability potential for this process. Another disadvantage is that different materials require different adhesives to properly bond them together. The interfacial bond between the two surfaces exhibits different properties than the rest of the material due to the fact that an adhesive typically behaves differently than the base material under the same applied stresses. The chemicals used in adhesives can also present environmental and health hazards. In addition, some plastics, are relatively chemically unreactive and do not effectively react with chemically reactive adhesives.
Solvents
Solvent bonding is also effective for some plastics. After the surfaces to be joined are softened by contact with the solvent, they are held together until the molecules in the plastic interlock across the bond line and the solvent evaporates. This process is simple and inexpensive, but may require long waiting times and can cause stress cracking due to the action of the solvent on the plastic. Times required to make a strong bond can be as long as four days, which significantly slows the manufacturing process. Solvents can also be very volatile, creating safety and health hazards. In addition, some plastics, (polypropylene, polyethylene, nylon) are basically insoluble to common solvents.
Welding Processes
Welding processes are the most widely used joining processes in high production applications because they are fast and versatile. Welding requires heating and some degree of softening or melting to form a bond. Thermoplastic polymers lend themselves to these processes because they can be quickly heated, formed, and then cooled to retain a new configuration or form. However, care must be taken not to degrade the plastic parts as repeated heating or overheating will eventually result in degradation and diminished properties.
As mentioned earlier, the three general classes of welding methods are thermal, friction/mechanical, and electromagnetic. Some of the more widely used processes from each of these classes will be discussed.
Thermal Methods
Thermal methods use heat generated by the tooling to soften or melt the surfaces to be joined. Hot gas welding, extrusion welding, hot tool welding, and infrared heating are thermal methods used for welding plastics.
Hot gas welding is a well established process, as it has been used to weld plastics for over 30 years. Its name explains the process well. A heated gas is used to soften the joint surfaces and a softened filler rod is used to fill the joint area and bond the surfaces together. The gas is typically just air, but an inert gas such as nitrogen must be used with some plastics to prevent oxidation. For a butt joint, the edges to be joined are typically beveled to increase the surface area for the filler rod to fuse with. This filler rod is heated and fed into the joint, similar to many fusion welding processes used for joining metals. However, unlike metals, the work piece and filler rod are not melted; instead they are softened just enough to allow them to fuse together.
The equipment for hot gas welding is inexpensive and portable making it ideal for specialty shops, field work, and repairs. This makes hot gas welding one of the most flexible welding processes available. However, this flexibility is offset by the fact that the process parameters must be strictly controlled to ensure a good weld. If the temperature of the joint is not controlled, the material can degrade or oxidize or a weld could be incompletely formed. As a result, the success of this process is almost totally dependant on the skill of the operator, making consistency a concern. It is also quite easy to trap defects in the weld during welding, resulting in a weakened weld. Because this is a manual process, it is also quite slow and not suited for high production.
Extrusion welding is very similar to hot gas welding. However, instead of using a rigid filler rod to fill the weld, the filler material is extruded directly into the joint. Gas is still used to heat the work pieces. The hot, extruded plastic and heated joint surfaces fuse together as they cool.
Hot plate or heated tool welding is currently considered the most versatile and simplest technique for effectively joining plastics. In this process, the surfaces of the parts to be joined are pressed against a heated tool and brought to their melting point. Once the surfaces are melted, the tool is removed and the surfaces are held together under carefully controlled pressure as the plastic cools. When done properly, the weld strength can equal the strength of the base material. However, care must be taken to reduce oxidation with some plastics, especially nylon. Dissimilar materials can even be welded with hot plate welding.
Hot plate welding equipment can be either portable or fixed despite the fact that the equipment is somewhat larger than equipment used in other joining processes. The portable equipment is used for welding large diameter pipes (up to 2xe2x80x2 in diameter) on site. Hot plate welding can join small and large thermoplastic parts.
Although hot plate welding is a forgiving process (requiring little surface preparation, even for complex geometries), there are disadvantages associated with it. Degradation and crystallization are concerns that are not fully understood, even today. Special considerations must be made concerning the tooling. The tool surfaces must be coated with a non-stick surface such as PTFE. These surface coatings limit the maximum use temperature to 500xc2x0 F., making this process unsuited for use with high temperature materials. This is also a relatively slow process with weld times ranging from a few seconds for a small part to 30 minutes for a large pipe.
Friction/Mechanical Processes
Friction welding is a very fast and effective method for joining certain part and joint geometries. The heat required to soften or melt the joint surfaces results from the friction generated between the joint surfaces as they are held under pressure. Relative movement between the two surfaces generates enough friction to melt the surface in less than three seconds. These frictional processes have much shorter cycle times than other processes and the strength of the resulting welds are close to the strength of the base material. Spin, vibration or linear friction welding, orbital or angular friction welding, and ultrasonic welding are all processes that rely on heat generated by friction to join parts. These processes have very short cycle times.
Spin welding is typically used for concentric parts that do not require any angular alignment. The surfaces to be joined are held facing each other while one of the surfaces is spun in relation to the other. Once the rotating part has come up to speed, the rotational force is disengaged and the parts are pressed together. Frictional forces convert the rotational energy into heat. The joint is completed as the spinning part comes to rest, having fused with the stationary part. Because this process uses relative surface velocities to generate heat, more heat is produced on the outside edges of parts. The difference in heat generated can lead to residual stresses across the weld. For this reason, joint design is a critical process parameter. This process is best suited for welding thin-walled, hollow parts, however, care must be taken when welding such parts since flash typically forms at the weld and can be difficult to remove from the internal surfaces of parts.
For joints requiring a specific part orientation about the axis of rotation, orbital welding, also known as angular friction welding, can be used. Instead of spinning the parts as in spin welding, the parts rotate back and force in an angular displacement. In this way, the orientation of the part geometries relative to each other is always known.
Vibration welding, also known as linear friction welding, operates on the same principle as spin welding except that the displacement is linear instead of rotational. This linear displacement is very small, on the order of 0.002-0.070 inches. When at the proper heating level, the parts are aligned, pressed together, and cooled. As with the other friction welding methods, this process does not require special surface preparations.
Each of these frictional processes is very fast and effective in their applications. Frictional processes typically produce a hermetic joint, free of voids and cracks. Because this process fuses together the surfaces to be joined, the resulting welds have strengths near that of the base material, with localized heating reducing the risk of degradation. These processes require minimal setup, have no associated consumables, and require little to no post process operations. They do, however, typically require specialized equipment.
Ultrasonic welding is one of the most widely used processes despite the fact that it is a relatively new process (Bauer 75). This process utilizes high frequency vibrations between parts to create friction and conditions of hysterisis. These conditions result in enough energy input to melt the parts at the weld interface. Because the energy is localized, a weld is produced without significant melting of the base material. This process offers several advantages which include: cycle times under 1.5 seconds, good quality control, efficient energy use, no consumables or toxins, and little operator training. Few processes compare with the speed of ultrasonic welding.
There are also some disadvantages associated with this process. Ultrasonic welding is limited to lap joints and staking operations. Staking is similar in concept to riveting in the way in which it mechanically joins parts. In staking, one part must have a post or nub that passes through an opening in the second part. This post is then softened using ultrasonic energy, while pressure applied by the ultrasonic tool mushrooms out the softened post so the post cannot pass back through the opening in the second part, thus forming the mechanical joint.
Because of the high energy levels required by this process, only small welds can typically be made with ultrasonic welding. This is why spot welding and staking are the most common operations performed with this process. Multihead machines allow larger parts to be welded. For example, welds can be up to 6 inches long when done with thin materials. The resulting welds exhibit a heat-affected zone which has decreased mechanical properties similar to that observed in metals. Ultrasonic welding is most effective with amorphous polymers.
Radio frequency welding is like ultrasonic welding, in principle. It also uses high frequency vibrations to disturb the molecules of the parts to be joined. This process has a smaller heat affected zone than ultrasonic welds, because it does not generate as much localized heating. It also works best with thin materials because thick materials dissipate the vibrations too much. However, there are safety considerations for radio frequency welding methods due to operator exposure to high frequencies and types of electromagnetic radiation. Also, several plastics are resistant to the frequencies used in radio frequency welding and cannot, therefore be joined by this method.
Some processes, such as lasers welding, use neither mechanical nor frictional forces to join plastics. Lasers rely on light radiation as the source of energy necessary to melt the material at the joint. Lasers are proving to be an effective method for joining plastics. Laser welding is a non-contact process so parts do not require robust fixtures to maintain proper part orientation during welding. Lasers offer extreme flexibility and low operating costs because they require no consumables and are low maintenance. Likewise, with the use of fiber optic lasers and robotics, it is possible to follow three-dimensional surfaces; something that most processes cannot do.
There are obviously a variety of techniques to choose from when joining plastics. Each process lends itself to certain applications. Available capital, space, trained operators, weld quality, volume of product, environmental concerns, necessary process flexibility, and especially joint design all influence the process choice. Unfortunately, most of the effective welding methods require expensive, specialized equipment. A process which could utilize existing machinery while offering lower costs, increased flexibility, limited joint preparation, increased safety, and the potential for automation would meet the needs of many industries.
Metal Welding Methods
Some of the welding techniques for joining plastic are analogous to certain metal welding techniques, such as hot gas welding. According, there has been an effort to adapt metal welding techniques to plastic materials. Among the metal welding techniques is a method referred to as xe2x80x9cfriction stir weldingxe2x80x9d (FSW) was originally developed for welding aluminum alloys is disclosed in U.S. Pat. No. 5,460,317, to Thomas et al.
The Thomas et al. method comprises causing a non-consumable probe or tool to enter the joint region and generating heat between the probe and opposed portions of the joint, which causes the opposed portions to become plasticized. The probe is removed or translated along the joint allowing the opposing plasticized portions to join and solidify. The Thomas et al. method is accomplished by two distinct methods, (1), using a flatted blade that reciprocates between the opposed portions, and (2) a rotating pin or bobbin spins between the joined portions as it advances along the joint. Preferably, the plasticized material is restrained from extruding from the joint region, for example by a shoulder above the pin which closely fits the work-piece surface and rotates with the tool, which shoulder also provides a frictional surface.
More particularly, the rotating pin method (2) for welding metals comprises plunging a nonconsumable, rotating pin into the joint line of two butted metal sheets of material. While the material is held very firmly in place, the rotating pin forces frictional contact and resultant heating. The pin extends from a rounded or radiused shoulder according to the required depth of the weld. The radiused shoulder rides on the top surface of the materials, generating frictional heat and holding the weld material in the joint. The frictional heating lowers the yield or flow strength of the material, allowing the pin to advance through the material along the joint line. As the pin advances, the softened material flows around to the backside of the pin where it reconsolidates. The weld size is determined by either the pin or shoulder diameter, depending on the setup. In the present application, this version using the rotating pin is what is meant by xe2x80x9cfriction stir weldingxe2x80x9d, is distinguished by the reciprocating blade welding method.
Efforts have been made to apply this method to other materials, and Thomas et al. does disclose the joining thermoplastic materials using his method, and shows examples of friction stir welding using a reciprocating blade method. However, use of the Thomas et al. friction stir welding processes with a rotating pin or bobbin have not been successful for welding plastics. The reason this method has not been successful for plastics is due largely to fundamental property differences between plastics and metal, including, the viscoelastic behavior properties, the relatively low melt temperatures of plastics, and other properties of polymeric materials. The result is usually a weld that has substantial voids from plastic material being ejected from the weld area, and a weld of low strength.
If friction stir welding could be applied to plastics, it would offer many advantages ranging from cost reductions to the ability to weld fairly complex, flat joints. Manufacturing costs would be lowered in several ways. Machine costs would be reduced because FSW would require minimal specialized equipment. A high quality vertical mill with minor modifications could be effectively used for this process. A company desiring to use this process could even convert an old mill to perform this operation. Machining mills are low maintenance, low cost, energy efficient machines. The fact that this is a machine tool operation means that the operators do not need special training in this process. Costs would be further reduced because there are no consumables. FSW would not require filler material, shielding gasses, or other consumables aside from electricity and the tools themselves. Special joint edge profiling would not be necessary and the joint surfaces would not require high tolerances or specially prepped surfaces, so prep work and setup would be minimal. Safety costs would be reduced because FSW is a safe and clean process as there is no arc emitting bright light or ozone. Similarly, there are no high voltages or gasses to worry about. Automation can further reduce costs while producing consistent high quality joints. The equipment could also be made portable, so economical field repairs could be done. The possible control and versatility associated with this process are appealing.
Objects of the Invention
It is, therefore, an object of the invention to provide a method and apparatus for friction stir welding for thermoplastic materials.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that does not leave voids in the weld that would weaken it.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that is applicable to multiple joint configurations.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that is safe, not requiring potentially hazardous solvents or radiation conditions.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that requires limited joint preparation, can be preformed by technicians with little training, and consistently produces high-quality welded joints.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that requires no specialized machinery, or expensive tooling to perform.
Another object of the invention is to provide a method and apparatus for welding thermoplastic materials that may be automated.
Further objects of the invention will become evident in the description below.
In has been found that friction stir welding for thermoplastics has been unsuccessful for various reasons relating the particular properties of thermoplastics. Unlike with metals, in thermoplastics the friction from a conventional rotating tool introduces insufficient energy into the thermoplastic materials to fuse the thermoplastic into a suitable joint. In the prior-art a shoulder can be provided to provide additional frictional surface and increase the input of energy, but the result is still unsatisfactory. Material in the weld region is ejected from the weld region, since the rotating shoulder functions more as an agent to eject the material than retaining the material-under shoulder. The loss of material results in a material deficient weld joint that is weak and contains voids.
It has been found that these problems in thermoplastics can be solved by two modifications to the FSW process; (1) providing a constraining surface which is stationary, or at least moves independently of the pin or rotating element, and (2) introducing energy by a system independent of the frictional energy produced by the tool.
With respect to (1) the providing a constraining surface which is stationary, or at least moves independently of the pin or rotating element, the constraining surface must have an independent movement from the pin for, as discussed above, if the surface rotates with the pin it will not constrain and inhibit ejection of the thermoplastic material. The constraining surface may be non-rotating or rotating independently of the rotating element. For example, it may be a non-rotating or slowly rotating shoe on the same or different axis from the rotating element, such as a roller moving on another axis transverse to the rotating axis of the pin. The rotating of the shoe may be in the same or in the direction opposite of the rotating element. The constraining surface also includes any suitable construction that provides the constraining function with the independent motion. This may include, for example, a roller moving on another axis transverse to the rotating axis of the pin. This construction may allow for a structure for rolling a surface over a portion of the weld forming region. The independent motion of the constraining surface is such that lateral forces are not translated into the rotating element from the constraining surface, which forces may lead to failure of the pin. In this manner the constraining surface is able to float as it is directed along the path of the rotating element, so that it does not induce lateral forces in the pin when the pin is stressed. The constraining surface can also function to assist in reconsolidation of the thermoplastic as it forms the weld behind the tool. It can also provide a surface for introduction of additional thermal energy.
With respect to (2) the introducing energy by a system independent of the frictional energy produced by the tool, the temperature in the weld region is therefore independent of the speed or rotation and travel of the tool. This allows for optimum conditions to not only provide the fusion of the thermoplastic, but also to control the flow of thermoplastic around the tool, preventing ejection, retaining thermoplastic in the weld region, and preventing voids. This also allows for a greater flexibility in design of the tool. A rotating shoulder to induce friction can be eliminated and the geometry of the tool can be designed to control the flow of the softened or melted thermoplastic, and/or to provide a surface to introduce additional frictional energy into the thermoplastic.
In an embodiment of the invention, the energy to soften the thermoplastic to a suitable fusion temperature is provided solely by the rotating and advancement of the element or pin. The geometry of the rotating element is specifically designed to produce the amount of frictional energy required to fuse the thermoplastic. The geometry may also include elements to control the flow of the softened or melted thermoplastic. In contrast to the rotating elements of the prior-art used in metal welding, the tools of the invention have geometry specifically designed to control and produce the frictional energy. This can by accomplished by any suitable geometry, preferably with a rotating element having non-circular cross-section. Such a cross-section would include any non-circular closed shape that functions to induce friction from the rotating of the element. Shapes that provide a cutting action to the thermoplastic material are preferably avoided. The non-circular cross-section may be polygonal, star-shaped, lobed, dumb-bell-shaped, ovoid, bladed, s-shaped, crossed, or a combination thereof. Illustrations of suitable rotating elements are shown in FIGS. 26A-1 to 26C-10, which show both cross-sectional and side views. Shown are non-circular shapes and pins (FIGS. 26A-1 to 26A-12, 26C-2 to 26C-8, 26C-11 to 26C-13) combined with variation or non-variation of cross-section of the shape along an axis of rotation (FIGS. 26B-1, 26B-13, 26C-9, 26C-10). The geometry of the rotating element may also include a surface with shear or friction producing protuberances, such as ridges, threads, flutes, grooves, or the like. The function of the construction is to produce frictional forces in the thermoplastic material to (input energy into the plastic material. The geometry may also have the function of controlling the flow of the thermoplastic material. For example, appropriately aligned threads may be provided to move melted or softened plastic material down into the weld from the surface. Such action tends to inhibit loss from of material form the weld region.
In an alternate embodiment, thermal energy is also introduced into the weld region by heating the weld region during the welding process. This may be accomplished, for example, by heating the constraining surface, and/or by heating the rotating tool.
The method of the present invention is suitable for welding any thermoplastic material, including many that are difficult to join by prior-art welding techniques, such as polyethylene. By xe2x80x9cthermoplasticxe2x80x9d material in meant herein to mean any plastic material that can be fused by heating the material. Examples include, but are not limited to polyethylenes, polyproplylenes, ABS, nylon, PEEK, and styrene. Depending upon the properties of the thermoplastic, supplementary thermal energy in addition to the frictional energy from the rotating element may be introduced. For low melting and/or high friction materials, no supplementary thermal energy may be required. However, for higher melting thermoplastics, or for xe2x80x9clubricousxe2x80x9d thermoplastics that produce inadequate friction energy (or those that tend to abrade) supplemental thermal energy may be required.
Friction stir welding of plastics according to the present invention has potential applications in almost any industry. FSW can be used to join large panels, repair faulty joints or cracked parts, to weld fuel tanks, container bodies, vessels, plastic window and door frames, to construct pipelines, to fabricate pipes, to join electrical component bodies and connectors, etc.
In summary, the present invention comprises a method and apparatus for forming welds in thermoplastic material. A rotating element is introduced onto a thermoplastic material and advances along the surface of the material on a path where the weld is to be formed. The speed of advancement and the rotational speed of the elements introduce frictional heat into the region around the element, i.e., the region where a weld is being formed, by fusing the thermoplastic material. The geometry of the rotating element is such to also introduce energy by frictional forces.
In addition to the frictional energy, thermal energy is optionally introduced into the weld-forming region from an outside energy source. The outside energy source could directly contribute thermal energy, or it could contribute some other kind of energy (e.g. mechanical or electromagnetic) that is converted to thermal energy by the material being welded. The combined friction, and outside energy introduced into the weld forming region fuses the thermoplastic material sufficiently to form a welded bond.
In addition to introducing energy by rotating the element, the element can be moved in a non-rotating movement that produces frictional energy in addition to that produced from the rotating of the element, and the advancement of the element along the path. The non-rotating movement may be vibration, oscillation, eccentric rotation, expansion, contraction, or a combination of these.
A constraining surface, such as a non-rotating shoe, bears on the surface of the thermoplastic material to contain softened or melted plastic and inhibit its expulsion form the weld forming zone from the rotational motion of the element. In addition, the constraining action of the non-rotating surface provides a forging pressure and assists in consolidation of the fusing thermoplastic after the rotating element moves on and the thermoplastic fuses and solidifies to form a welded joint. The pressure applied by the constraining surface is preferably sufficient to assist in consolidation, but insufficient to penetrate the plate into the thermoplastic material to weaken the weld. The constraining surface should be able to float with the rotating element so that it does not induce lateral forces in the element when the rotating element is stressed, particularly by lateral forces. The constraining surface can translate generally with the element as it travels down the path of the weld, but is does not rotate with the element. In this way it cannot contribute to expulsion of thermoplastic from the weld forming zone. The non-rotating surface of the constraining surface may be any suitable configuration. For example, it may be flat, not flat, cylindrically or spherically concave or convex, or dome shaped, adapted so that it will function to capture material and to apply the consolidation pressure. (See constraining surface shapes in FIG. 27A to FIG. 27E-2) In particular, there may be special geometries that are developed to affect the compression of the plastic and hence the final properties of the weld, such as the finish of the weld.
The constraining surface may be non-rotating or rotating independent of the rotating element. These include, a non-rotating or slowly rotating shoe on the same or different axis from the rotating element. The constraining surface may also include any suitable construction and provides the constraining function. This may include, for example, a roller moving on another axis transverse to the rotating axis of the pin. Suitable constructions for a constraining surface are illustrated in the examples, and in FIGS. 27A to 27E-2.
In a preferred embodiment of the invention, the additional thermal energy introduced in the thermoplastic incorporates the constraining surface, whereby the constraining surface is heated by any suitable means and heat is transferred through the contacting surfaces of the constraining surface and the thermoplastic material. The constraining surface may also be shaped to accommodate the input of the thermal energy, in any portion of the welding zone. The constraining surface may also, at least in part, be insulated to reduce the dissipation of heat from the weld region.
Alternately, the thermal heat can be introduced into the weld-forming zone by heating the rotating element.
Suitable heating methods for the constraining surface or the rotating pin include, but are not limited to, sonic heating (including ultrasonic and subsonic), resistance heating, conduits using a hot heat transfer fluid, inductive heating, and hot gasses or flames. Systems for introducing thermal energy include, but are not limited to, inductive heater, resistance heater, a gas flame, fluid heat exchanger with a heated fluid, a sonic generator, a chemical reaction, or a combination of the above.
In addition, the thermal energy source may introduce thermal heat into the welding zone by inducing thermal energy in the thermoplastic, such as through heated gas streams, ultrasonic or radio frequency waves, using any suitable method.
Thermal energy may also be removed from any portion of the weld forming region to control the temperature of the heated thermoplastic materials or assist in consolidation of the heated and fused thermoplastic material. This may be accomplished by cooling the rotating element and/or by cooling at least a portion of the constraining surface.
The piece or pieces of thermoplastic material being welded should be held firmly during the welding operation to hold adjoining pieces firmly against one another and allow the constraining surface to apply pressure. Depending upon the type of welding joint, a backing plate may be required to so that the pressure can be applied. Certain lap joints, including pipe joints, may be self-backing and do not require a backing plate because the rotating pin may only partially penetrate into the underlying piece.